Power line carrier telephone system



1932' H. B. ARNOLD POWER LINE CARRIER TELEPHONE SYSTEM Filed Aug. 10,1929 2 Sheets-Sheet 1 INVENTUR H. E). ARNOLD W/ ATTORNEY Aug. 23, 1932.ARNOLD 1,873,255

I POWER LINE CARRIER TELEPHONE SYSTEM Filed Aug. 10. 1929 2 Sheets-Sheet2 H I V I it ATTORNEY Patented Aug. 23, 1932 UNETED STATES E RY R.ARNOLD, OF NEW YORK, N. Y., AssrGNO To" BELL TELEPHONE LAB RA- TORIES,INCORPORATED, OE NEW YORK, N. Y.,'A CORPORATION OF NEW YORK POWER,LINECARRIER TELEPHONE SYSTEM Application filed August 10, 1929. SerialNo. 384,928.

' carrier current signal system effecting communication between stationsby modulated carrier current and seleetlve calling by code signals ofunmodulated carrier current that.

shallhave electromagnetic call means at each station responsive only toa code signalpf carrier current free from modulat on wlth voicefrequency currents. Another object of the invention is to provide apower line carrier-current signal system that shall effect communicationbetween stations by modulated carrier current and selective calling bycode SlgIlZtlSOfCZLIIlGI' current free from any modulation with voicefrequency currents and that 'shall'employ a differential relay at eachstation to insure operation of the call selector only when a code asignal stations is effected'by voice frequency currents modulated withcarrier current and selective calling oft-any station is effected bytransmitting a codesignal of impulses of one. wave within the voicefrequency range modulated with carrier current. The patent to C. N.Nebel No. 1,733,553 dated October 29,

1929, discloses a power line carrier current signal system wherein thesame carrier wave is employed for communicationin opposite directionsbetweensignal stations. In such system only one side band is'transmittedOver the power line during communication between signal stations.Selective calling of any station is effected bytransmitting apredetermined number, of impulses of one wave within the voice frequencyrange modulated with a carrier wave. The number of impulses of modulatedcarrier current required to operate a selector at any station ismadesufficiently high andthe timing adjustment is arranged to preventoperation of a selector during communication' between" stations.

' mitting circuit is in use. p

The application of J D- Sarros SerialNo.

384,87 6 filed August 10, 1929, discloses a pow or line carrier currentsystem employing the same carrier wave for transmission inoppositedirections and wherein the carrier wave and both side bands aretransmitted during communication between any two signal, stations. Thepresent invention is particularly adapted for use in the systemdisclosed in the above mentioned application of J. D. Sarros. The powerline carrier current signal system employed to disclose this inventionis substantially the same as the system disclosed in the above mentionedapplication of J. D. Sarros and the selective calling circuits shown intheapplication of J. D. Sarros, Serial No. 384,876 are similar to thecalling circuits disclosed hereinafter.

In the system employed to disclose the present invention a plurality ofcarrier current signal stations are connected to a power line in anysuitable manner as for example, by condensers in the manner disclosed inthe above mentioned patent to C. N. Nebel, No. 1,733,553. Each signalstation is provided with a transmitting and receiving channel. Thetransmitting channel comprises a threeelement thermionic modulatortubewhich is supplied with 'carriercurrent from a'suitable oscillator andwith voice frequency currents from the operators telephone set, or fromthe onic detector tube for separatingthe voice frequency currents fromthe carrier waves 'duringcommunication between two stations.

The detector tube is connected through suitable amplifying means to thereceiver of the operators telephone set. Suitable duplex controlcircuits are provided for impressing a negative blocking potential onthe grid of the modulator tube when the station is idle, or thereceiving circuit is in use and for im-- pressing a negative blockingpotential on the grid of the detector tube when the transtelephone setof an operator at an extension any operation of the differential-relayand Selective calling circuits are provided for selectively calling anydistant station. Means operated by the selector key is provided foroperating the duplex control circuits when calling a distant station soas to impress a negative blocking potential on the grid ofthe detectortube and remove the negative blocking potential from the grid-of themodulating tube in the transmitting channel. When transmitting a codesignal from any station, unn'i'odulated carrier current free from anymodulations with voice frequency current is transmitted over the line tothe distant station. A differentially wound relay is provided "forgoverning the operation of the selector at each signal station. lay ispreferably polarized. One coil of the 'differential'relay is energizedfrom theout- The differential re put circuit of the detector tube toprevent the selector when carrier currentmodulated with'voice frequencycurrent is received over the power line. A second coil is provided onthe differential relay which is energized whenever carrier current isreceived overthepower line. The differential relay isionly operated toeffect operation of the selector when a code signa'lof unmodulatedcurrent free from any modulated carrier 'current'is received. If voice"frequency currents and f 'modulated currents are simultaneously received over the power line, the two coils of the differential relayneutralizeeach other and insure against any operation of the selector.By such means it is possible to prevent any operation of the selectorduring communication between any two stations. In the application ofCharles'N. 'Nebel, [Serial No. 384,878 filed August 10,1929; isdisclosed carrier current g wherein thermionic vacuum tube calling cir--cuits are provided in a system similar to I that disclosed in the abovementioned patent to C. N. Nebel, No. 1,733553. j

. Figures 1 and 20f the accompanying drawings disclose a signal stationfor a power line carrier current telephone system constructed inaccordance with the invention.

Referring to the drawings a sigiialstation I having a transmittingchannel 1 and a retransformer 87a and a hybrid coil 8. The hybrid coil 8is provided with the customary balancing impedance 9.

,65 circuits lO are provided for blocking the" ceiving channel 2 isconnected to a power. line comprismg conductors 3 and 4 by means of acoupling circuit 5. The coupling circuit 5 ispreferably ofthe typedisclosed in the above mentioned patent to 'C. N. Nebel No. 1,733,553.An operators telephone set comprising a transmitter 6 and a receiver 7,

is connected to the transmitting channel 1 and the receivlng channel 2by means of a Duplex control transmitting channel when the 'st-ation'isidle plex control circuits.

later time 11 and signal systems and 20.

and when the receiving channel is in use and for blocking the receivingchannel when the transmitting channel is in use.

The transmitting channel 1 comprises a c three element thermionicmodulator tube 11,

which is connected to the hybrid coil 8 by means of a transformer 12,Anoscillation generator 13 which is preferably of the thermionic type isconnected to the input circuit of the modulator tuhe'll by'nieans'of atransformer 14. The carrier'wavedeveloped by the oscillationgenerator 13is modus lated' with the voice frequency currents received from theoperators telephone set.

Plate potential for the modulator tube 11 'is'supplied'from a battery 15through a retard coil 16. Grid biasing potentialfl for blocking the tube11"is obtained from-the drop across a resistance element 17 in theduplex control circuits 10. Aneziplanzttionof the operation of theresistance element-'17 in controlling'the modulating tube=.11 will tors3and 4;, The grids of the tubes 19*and 20 are supplied'with negativeblocking potential from the resistance element 17 in-the same manneras'the grid for the modulator tube 11. Plate potential for the tubes 19'and 201s provided by a battery 23. A battery fld supplies filamentheating current for the'm'oduthe two amplifier tubes 19 The'receivingchannel 2 comprises a'p'otentiomet'er 25 and a band 'pzssfilterQGWhi'chare connected to the powerline-conductors 3 and 4 throughthe couplingcircuits 5. The bandpass filter 26 which selects 'the carrier wave andside bands characteristic to thesystem' is connected to adetect0r'tube27 by means of the transformer 28.- The detector tube 27 iscoupled to twoamplifier tubes 29 and 30 in push-pull relationship-bymeans of a'transformer'3l. Gridb iasingpotential for couplingcircuits-5to the powerline'c'ondu'cblocking the detector tube 21 andthetwo amplifier tubes 29and3Q is suppliedby the "potential dropacross'a resistance element '49 in the duplex control circuits 10.Filament heating current for the detector tube/2T and the amplifiertubes 29 and 30 is supplied from a battery 32, Plate potential forthe'dete'ctor tube 27 and the two amplifier tubes 29 and30 is suppliedbya battery 33.- I y The output circuits of the amplifier tubes 29 and30are connectedby a.' transfo-rmerv isl,'

a low pass filter35and a tTan'sformerBG tQ a" mas-e thermionicamplifier'tube The low pass -filter permits the voice frequency currentsdetected by the tube27 to freely'passtherethrough. The amplifiertube'37f isdirectly connected to the hybrid coil 8. Thesecon'd arywinding of thetransformer 36 is not 'only connected to the input circuitof the amplifier tube 37 but is also connected to an amplifier tube 38.The amplifier" tube .38 is connected by a transformer 39 to a rectifier'Etube 40 which: governs the duplex control c ir-. fcu'its when thereceivingcha'nnel is in use, Plate potential for the thermionic tubes 37and'38 is received from a battery 41. v Filamentheating current for thetubes 37, 38 and 40 is obtainedfrom a battery 42. The heating current,circuit may be traced from the battery 42through resistance element 43and 44, filament of the tube 38, filaments of the tubes 37 and40,'filament of a thermionic tube 45 in the duplex control circuits .10and ground return' to the battery 42. Grid bias for the tube 38 isobtained from the drop across the H resistance element 43 and Grid biasfor the tube 37 in a like manner is v to ground. The resistance element48 controls the biasing potential impressed on the grids of the tubes 46and 45 in the duplex control circuits for blocking the transmitting apolarized differential relay 55.

former 50in the output circuit of the detector channel when thereceiving channel is in use. The output circuit of the detector tube 27is connected by a transformer 50 to an amplifier 51. The amplifier tube51 is connected by transformer 52 to a. thermionic tube 53 which in turnis connected to a winding 54 of The trans tube 27 collects the highfrequency carrier wave whch may 'pass through the detector tube. Platepotential for the tube 51 is supplied by battery 33 and plate potentialfor the tube 53 is supplied by battery 56. Grid biasing potential andfilament heating current for the tubes 51 and 53 are supplied by thebattery 32. A more complete description of the operation of thedifierential relay '55 and the thermionic tubes 51 and 53 willbe givenwhen reference is made to 'theselective calling circuits.

The duplex control circuits 10 comprise the thermionic tubes 45 and 46in push-pull relationship which are connected by atransformer 57 andpotentiometer 58 to the transmitting channel 1. The output circuits ofplifier tube 47 is connected by a transformer 60 to arectifier tube 61.]late,potential for the tubes 45, 46 and 47 obtained from thebattery'41. Grid biasing: potential for the tubes 45and 46 asheretoforeset forthis' V obtained the drop across the resistance element48." .The filament heating circuit for the tubes 46, 47, 61 and athermionic'tube 162 extends from a battery 42, through the 4 filamentsof the tubes 62. and 61, a resistance element 63, filament of the tube47," filament of thetube46'and ground'return to thefbattery 42; Gridbiasing potential for the thermionic tube47 is obtained fro1n 'thefilament heating circuitl The plate I circuit through the rectifier tube61 may be traced fromj the filament of thefrec tifiertube through thefilament of the tube/47, filament of the tube 46, ground returnconductor, {battery 32, resistance element49," and the "secondary of thetransformer 60to the plate or. the tube'61. V V

,. The three element thermionictube'62'controls the potential impressedon the resistance element 17 from a battery 65. The potential impressedon the grid of the tube 62 is controlled by the resistance element 49.When the transmitting channel is 1n service,

the potential drop across the resistance element 49 places a highnegative potential on the grid of the tube 62. The blocking 'of tube 62-prevents impressing of a blocking negative potential on the gridsof themodulator' tube 11 and' the two amplifier tubes 19 and 20 from theresistance element 17. When the station is idle, and when'thereceivingchannel is" in use no potentialis impressed on the grid of .the tube62from the resistance elemen't49. Accordingly when the station is idleand when the receiving channel is in use, negative blocking potential isimpressed upon the modulator tube 11 and lthe'amplifier tubes 19 and 20.The grid circuit for-the modulator tube 11 may be traced from the gridof the modulator tube 11 through the secondary winding of the.transformer 14,

.The' differential relay 55, which is of the 'polarized'type,controls aselector 66 for operating a call bell 67 atthe operators station or acall bell (not shown) on an'extension station. The 'difi'erential relaycomprises the coil 54 which is energized by rectiprovided for the V fiedcarrier current impulses, afwinding 68,

which is energized by rectified voiceffrequency currents and a winding69 .which'is energized according to the operation of a selector key 70.The selector key 70 may be jof'anyfsuitable type such as the masterimpulse transmitting device disclosed in U. S. patent to J. Cr Field,#1,354,814,'.dated October 5, 1920. The differential relay 55 ,7 e 4Uponoperation ofthelimpulse transmit- :isiprovided with an armature 71operating between contact members 72ra'nd '7 3,cf:or conrolling theselector 66 from the battery 165.

A? poten'tiometer 74, which is connected rni 'nals thereof connectedftothe contact elemerits '72 Yand73. "The armature 71 of the differentialrelay is connected through a con- I i lidenser 75r and a :coil of theselector 6'6 toa mid-tap 'ofthe potentiometer 74. "The circuit forenergizing thewinding 69 of the dif ferentialrelay extends from agrou'nded'bat 'tery .76 through *the coil69, impulse transmittin g key70,"and ground return to the battery '76. .Puls'ingof the differentialrelay jbvfthe :selector key 70 serves to vibrate the jarmature 71" forimpressing impulses' from the battery 65 on the selector "coilto effectoperation of thefselector 66 in accordance withthe code signal beingtransmitted;

thereof connected by transformer 7 8 "to the .420 thebattery 56. Thecoil 68 ofithe difierential foutput circuit of the lots pass filter '35.[The output circuit of vthe rectifier. tube '7 7 is'con-' nected to thecoil 68 of the di'fie'rential relay 1.55. The, energizing circuit forthe. coilv 68 of .the diiierentia l relay may be traced'from thegrounded battery '56 through the armature 79 of a relay80, coil 68,plate 'and filatment of the tube 77 and ground returnto Qpow erline,ithe winding 6 8 as well asIthe --winding 754;:is energized toinsureaga-inst.

any operation of the differential relay :and

the selectorj- No, operation-of the selector can take place duringcommunicationbetween r-stations.

,tingKkeyV-TO for transmittinga .code calling signal overjthepowerline,-a..pulsing relay V 81 isenergized by a-circuitzextending from cuitacross the transmitting channel. :and ,an

"thecbatter y"56 through a slow releasing relay battery 82. The ;pulsing relay :81 isprovided with an armature 83-forplacingashunt-cirarmature S L for completing a circuit from .85.Thearmature 83 .ofrthe relay 81 inter- -mittently places a shuntcircuitiacross the transmitting channel in accordance withlthe "codesignal to transmit impulses of carrier :current over the power line.vThe relay 85,

I -tur'e SGfOrcQmpIeti-ng -a circuitffrom the bat-- Itery 76 throughiasignal-"tone relay 87. I

which is slow releasing, is held in anopersated position duringvpulsingof the relay 1.81. 'The relay 85 isprovided with an armaac'rossthe/battery '65,- has two opposite ter-V A rectifier tube 77 has theinput circuit Thesignaltone.relay87 serves when trans-- mitting aselective call to a distant station,

for inipressingiliow freq en y curr tf om I anvosci'llatQii/88 uponitheduplex control cilr supplies a current of sixtycycles, isconnectedprimary winding vof the transformer 57;

When the oscillator '88 isconnefcted to-gthe duplex 'con'troIcircuits,it is 'apparent that potential will-befproduced across the -resiStaIlQIe element 49' for impressing negative blocking potential on the gridsof the xd ifli tor tube- 27 and thea-mplifier"tubesQQand '30 in thereceivingc'hanne'l. Furthermore it is-appar I o ent that the negativegrid vpotential impressed flupon the tube '62 jifrom the resistanceelement 49v ,Wlll insureagainst any blocking of the modulator tube '11and the amplifieritubes 19' and 20 in the transmitting channel.

flommum'cating with a-dz'stdnt station V t'ion voice frequencycurrentsfromthe operators telephoneset are modulated with carriercurrent from the oscillatorlg in themod ulatortnbe ll. Thecarrierwaveland both side bandsfare amplified by the tubes-19rand 20 andthe power vamplifier 22 fo'rfltransmission over the power'line to adistant ofiice.

Voice frequency currents are also supplied to "the transformer 57'in'the d'up'lex'bontrol circuits 10. The amplifier tubes-i5 and'46inpush-pull relationship and the amplifiergtube e7 are operated forsupplying current tothe rectifier'tube 61.; The currentrectified-by thetube a 61 -;impresses potential across the resistance element e9. Thepotential across the resistanceelement 49impresses a negativeblockingpotential on the grid of thetube 62 to insureagainst'anyblocking ofthe'modulator tube '11 and the two amplifier ftubies '19'and2O in theftran'smittin channel. More-' "over, thejpotential across t'eresistance'ele- V ment 49 impressesnegative' blocking1potential on thegrids of the? detector'tube 2.7 and-the v amplifier tubes 29lganjd -30to insurecblocking of the receiving channel 2. Atthis-timethe by thearmature 89 of the relay 8? toitlle h jWhen communicating" witha distantstareceiving channel is not in useand accordingly suifi'cient potentialis'anot impressed acrossithe resistancerelement 48 :forblocking thetubes circuitsf During communication withv a "distant stationrarriercurrent .modulated with .voice 'ifrequen'cyfcurrent's is receivedover-thepower 45 and 46in the duplex control line for energizing th'ereceiving'channel 2. The received modulated current lpasses through thebandpass filter 26 to the detector :tube 27. v The detector tube 27separates the voice frequency currents ;from 'the carrier current andllpplies voiceifrequency currents through. the amplifier tubes 30and-"29, transformer 34', low pass filter 35,1tfansformer 36,1andthetube 37'to the hybrid coil "'[Thehybridcoi18is'eonnectedfgbya-transformer 87a to the receiver of the 'operators "telephone set. a

The amplifier tube 38 and the rectifier tube 40 areoperated by the voicefrequency curg rents received from the low'pass filter 35 for impressingpotential across the resistance element 48. The potential across theresistance {element 48 impresses negative blocking potential on thegrids of the tubes 45 and 46 in'the duplex control circuits. Theblocking of the tubes 45 and 46 insures against imi- -pressing potentialacross the resistance e]e.- ment 49. Inasmuch as there is no, potentialacross the resistance element 49, the tube 62 sistance element 17impresses negative blocking potential onthe grids of the modulator tube11, and the amplifier tube 19 and tube 20 in the transmitting channel.No nega- 20 tive blocking potential can be impressed on the outputcircuit of the low pass filter 35..

Consequently at this time no operation ofthe diflerential relay can takeplace.

Selective calling circuits 9 In transmitting a selective calling signalto a distant stationpthe key 70 is operated for pulsing the relay 81 inaccordance with the code signal to be transmitted. The relay 81 operatesthe armature 83 for intermittently placing a shunt across thetransmitting channel to intermittently transmit impulses of carriercurrent from the oscillator 13. 7 At the sametime the armature 84 of therelay 81 eifectsxoperation of the slow releasing relay-85. Theslowreleasing 'relay 85 oper ates relay 87for impressing a signal toneof low frequency currentfon the duplex control circuits 10. The. lowfrequency current impressed 'on the duplex control circuits serves toproduce a potential drop-across the resistance element 49. Suchpotential drop across resistance element 49 blocks the tube 62 andinsures against any blocking of the tubes in the transmitting channeland insures theblocking of the tubes in the receiving channel. Thewinding 69 of the differential relay 'is energized in accordance withthe pulsing of the relay 81 for operating the se lector 66 according tothe selected calling signal being transmitted. k

.If, a selective calling signal is received from a distant station, thewinding 54 of the dnferential relay 55 is intermittently ener- .165gized for operating the selector 66 accordis not blocked and thepotential across the re ing to the received signal. Inasmuch'as theselective calling signal comprises impulses of unmodulated carriercurrent, the winding 68 of the differential relay will not be energized;Differential relay 55 is prefer'ablyfa 5 polarizedrelay in'ordertoprevent operation of the relay in case the voicefrequencysup pliedto-the winding 68 are weak compared vwith the high frequency impulsessupplied to the winding-54. The selector 66 when operated by; thedifierential relay 55, completes a circuit from the grounded batterycircuit 7 6 through the switch arm 90, contact member 91, call bell 67.and ground return to the battery 76. A contact member 92 on the selector66 may be connected to the call bell at an extension station (notshown).

Modifications in the system and in the arrangement and location of partsmay be made within the spirit and scope of the invention andsuchmodifications are intended to be covered by the appended claims Whatis claimed is: I I 1. e In a carrier. current signal system, a signalstati0n,-m'eans at saidstation for effecting communication over thesystem by modulated carrier currentand for eifecting selective callingby a code signal of carrier current, a selector at said station, andmeans comprising a differential relay having one winding energized inresponse to the receipt of a code signal of carrier current impulses foroperating said selectorand a second winding energized in response to thereceipt of modulated current forpreventing operation p 9 of theselector: I

2. Ina carrier current signal system, a signal station, means at saidstation for effecting communication over the system by modulated carriercurrent and for effecting 5105 selective calling by a code signal ofcarrier current, aselector at said stati'on, and means comprising apolarized differential relay for operating said selector in response tothe re- .ceipt of a code signal'of carrier current.

impulses and for preventing operation of the selector in response to'thereceipt of separated impulses of modulated carrier current.-.

3. In a carriercurrent'signal system, a signal station having a receiverand a switchhook therefor, meansat said station for com municating overthe system by modulated carrier current and for effecting selectivecalling by transmitting a code signal of unmodulated carrier current, aselector at said station, and means comprising an electro-magnetic relayresponsive before thereceiver is removed from the switchhook' tounmodulated carrier current impulses received over the system andunresponsive before the receiver is removed from the switchhook toseparated impulses of modulated carrier cur rent received over thesystem for governing the operation of said selector.

ting channel for transmitting a carrier wave modulated with voicefrequency current to effect communication over the power line and fortransmitting a code signal of unmodulated carrier current impulsestoeifect selective calling, a" selector at said 'station,'and

means in said receiving channel comprising "a polarized difierentialrelay for operating said selector in response to the receipt of a codesignal of'unmodulated carrier current impulses and for preventingoperation of the selector in'response to the receipt of separatedimpulses of modulated carrier current.

5. In a power line carrier current-{signal system employing modulatedcarrier current for communicating bet-ween stations and unmodulatedcurrent to selectively call a st-ation, a power line, asignal stationconnected to the power line, a receivera-t said station having aswitchhook therefor, a selector at said station, and means comprising anelectro-magnetic relay for operating the selector in response toqthereceipt ofa code signal of unmodulated carrier-current before thereceiver'is removed from the switchhook and "for preventing operationof'the selector in response'to-the receipt of carrier. current modulatedwith voice frequency current before the receiver is'removed from theswitchhook.- f i 6.- In a power line carrier current signal system, apower line, a signal station having a transmitting anda receivingchannel connected to said line, means in said transmit ting channel fortransmitting 'a carrier wave modulated with voice frequencycurrent toeffect communication over thepower line and relayfor operating theselector in response to the receipt of a code signal of unmodufortransmitting a codesignal of unmodu- 'lated carrier current to effectselective'calling, a selector at said station, and means in saidreceiving channel comprising a differensponse to the receipt of acodesignal; of carrier current and for preventingoperation of theselector in response to the receipt of modulated carrier current. I

7. In a power line carrier current signal system, a power line, a signalstation connected to said power line, a selector atsaid station, andmeans comprising a differential lated carrier current and for preventingoperation of the selector in response to the re-' ceipt of carriercurrentmodulated with voice frequency current.

8. In a power line carrier current signal system, a power line, a signalstation connected to said power line, a selector at said vstation, athree element thermionic detector tube for-obtaining voice frequencycurrentsfrom received modulated current, andmeans comprising adifferentially wound relay for the detector to prevent operation of thelector by the relay when modulated currents are received.

9. In a carrier current signal system, a signal station having atransmitting channel and a receiving channel, means at said station forcommunicating with a distant station by modulated carrier current andfor selectively calling another station by transmitting a code signalof'unmodulated carrier current, means at said station for blockingthereceiving channel when the transmitting channel is in use and forblocking the transmitting channel when the receiving channel is in use,a selector at said station, and meanscomprising an electro-magneticrelay responsive to unmodulated carrier current for governing theoperation of saidselector.

10. In a power line carrier current signal system, a power line, asignal station having a transmitting channel and a receiv ng channelconnected "to said power line, means at said station for communicatingwith a distant station by modulatedcarrier' current and for selectivelycalling another station by transmit ting a code signal of "unmodulatedcarrier current, means at said station for blocking the transmittingchannel when the receiving channel is in use and for blocking thereceiving channel when the transmitting channel is in "use, a selectorat said station, and means comprising a differentially wound relayresponsive to unmodulated carrier-current and unresponsive tomodulatedcarrier current for governing the operation of said selector.

7 11. In a carrier current signal system,- a signal station having'a.transmitting anda re- .ceiving channel, a three elementthermionicmodulator tube and a source of carrier waves in said transmittingchannel, means for sup plying voice frequency to saidtube for modulation with said carrier current tocommunicate with a distant station,means for transmitting a code signal of said carrier wave forselectively calling a distant station, a detector in said receivingchannel for obtaining voice frequency currents from the receivedmodulated currents, a selector, a relay having two differential windingsfor operating said se lector, means for energizing one of said windingsby voice frequency currents in the detector output circuit and forenergizing the other winding when carrier wave impulses are received toeffect operation of the selector only when carrier current free frommodua lation with vvoice frequency current is received. 12. In a carriercurrent signal system, a signal station having a transmitting and areceiving channel, a source of carrier waves, a three element thermionictube in said transmitting channel for modulating voice frequencycurrents with said carrier waves to communicate with a distant station,a three element thermionic detectir tube in said receiving channel forobtainingvoice frequency currents from the received modulated currents,means comprising duplex control circuits at said station for applyingnegative blocking potential to the grid of said modulator tube when thestation is idle or the receiving'channel is in service and for applyinga negative blocking potential to the grid of the detector tube when thetransmitting circuit is in service, means comprising a selector key foroperating the duplex control circuits to remove the blocking potentialfrom the modulator grid and for transmitting a code signal ofcarriercurrent to selectively call a distant station, a selector, and meanscomprising a differential relay having one winding energized by thevoice frequency currents in the detector output circuit and anotherwinding energized when carrier Waves are received tooperate the selectoronly when carrier current free from modulation with voice frequencycurrent is received. In witness whereof, I hereunto subscribe my namethis 8th day of August, 1929.

HENRY B. ARNOLD.

